What you can't do is go abroad with a G-reg and your FAA license (a few countries accept it), but as long as you stay in the UK you should be fine
There is no teritorial limit. The licence is valid on a G reg wherever it might be. The ANO Article 62 no longer applies to EASA aircraft but does apply to Non EASA aircraft. There is an Exemption in ORS4 No 998 which again does not impose any teritoroal limit and extends the validation to 8th April 2015. After that a validation can be obtained for 1 year using form
SRG1118 You will also require an FCC Radiotelephone Certificate.